Natural rubber in an unmasticated state, ordinarily having a weight-average molecular weight of approximately 2,000,000, is extremely hard and, accordingly, it can neither be kneaded nor dissolved. In order to allow the natural rubber to be kneaded, it is necessary to decrease the weight-average molecular weight thereof to the range of approximately from 800000 to 1300000 by, even though slightly, applying shearing. Further, when the natural rubber is used after being dissolved in an organic solvent, since it is not dissolved in the organic solvent unless the weight-average molecular weight thereof is decreased to be 800000 or less, it is necessary to decrease the molecular weight thereof by masticating it to such an extent as it can be dissolved.
Further, synthetic rubber has a weight-average molecular weight of approximately from 300,000 to 1,000,000, which is lower than that of the natural rubber. Although low molecular weight rubber having a weight-average molecular weight of approximately from 10,000 to 50,000 is available in the market in the synthetic rubber, intermediate molecular weight rubber having a weight-average molecular weight of approximately from 100,000 to 200,000 is not available in the market. Therefore, it is sometimes required to decrease the molecular weight as the synthetic rubber having such high molecular weight as described above to the intermediate molecular weight.
As for methods for masticating rubber, there is a physical method in which mastication is performed such that the molecule cleavage is physically caused in the rubber by applying shear by means of a batch-type kneader such as a mixing roll, a Banbury mixer or a pressure kneader. Ordinarily, this physical method is in many cases performed by adding a mastication accelerator. Further, besides such physical method as described above, there is a chemical method in which a peptizing agent is added to a solvent to generate a radical which, then, chemically causes the molecule cleavage.
When the physical method is performed without using the mastication accelerator, it takes time and, particularly, when it is intended that the weight-average molecular weight is decreased down to 400,000 or less, it takes a large amount of time. This is because, when it is intended that the weight-average molecular weight is decreased down to 400,000 or less by the physical method, a viscosity is unduly decreased and, accordingly, it becomes hard to apply the shear. On this occasion, when the mastication is performed without using the mastication accelerator, it takes a still longer time.
In the chemical method, the molecular weight is controlled to a desired value by adjusting an amount of molecule cleavage by means of an amount of the peptizing agent to be added (Patent Documents 1 to 3). Specifically, the rubber is loaded in a dissolving kettle containing an organic solvent or the like and, then, added with the peptizing agent to cause a molecule cleavage while stirring and, as the molecular weight is decreased, the rubber comes to be dissolved in the organic solvent and, then, various types of compounding materials are mixed to the rubber in such a manner as they are dissolved in the resultant solution, to thereby obtain a dissolved article. Thereafter, this dissolved article is formed in a sheet shape by using a casting technique or the like and, then, put in a drying oven, to thereby remove the organic solvent therefrom.
However, this method requires a large amount of energy and, also, an expensive system for recovering an evaporated organic solvent.    [Patent Document 1]            JP-A No. 10-101736;            [Patent Document 2]            JP-A No. 10-102023            [Patent Document 3]            JP-A No. 2001-55548        